Generally, an optical semiconductor device is constructed by sealing with resin an optical semiconductor element such as a light emitting diode or a photodiode, by use of silicone resin, epoxy resin or like resin. With blue LED and white LED that have been recently given attention to, the sealing resin thus used is required to have not just optical transparency and light resistant transparency but also properties such as (i) heat resistant transparency that can resist heat generation while passing through electricity and (ii) crack resistance so that no crack generates in the sealing resin due to thermal impact upon mounting or the like.
If a conventional epoxy resin composition is used as the sealing resin, the heat and light resistance is insufficient, and brightness deteriorates in a short period of time. Accordingly, as means for improving light resistance, techniques that use an alicyclic epoxy resin as follows have been made available (e.g., Patent Literatures 6 and 7).

However, even with the technique that uses the alicyclic epoxy resin, the heat resistant transparency is still insufficient, and further improvement in heat resistant transparency has been strongly requested for.
Silicone resin is known as a sealing resin that has excellent heat and light resistant transparency as compared to the epoxy resin. However, although the silicone resin has excellent heat and light resistant transparency as compared to the epoxy resin, the silicone resin is weak in strength. Due to this weak resin strength, a hard silicone resin cured product easily cracks upon thermal impact testing.
In response to such a problem with the silicone resin, various curable compositions have been proposed, which include, as essential components: an organic compound that has, per molecule, at least two carbon-carbon double bonds that are reactive with an SiH group; a compound that has at least two SiH groups per molecule; and a hydrosilylation catalyst (for example, see Patent Literatures 8 and 9).
For example, in view of attaining good compatibility within the curable composition and making production of a cured product easy, Patent Literature 8 discloses a curable composition that uses a compound obtained by carrying out hydrosilylation reaction to an organic compound including at least two carbon-carbon double bonds per molecule and a cyclic polyorganosiloxane having at least three SiH groups per molecule, as the compound that includes at least two SiH groups per molecule.
Moreover, Patent Literature 9 discloses a technique which improves breakage of a molded cured product, by preparing a curable composition using, as the compound including at least two SiH groups per molecule, a compound obtained by carrying out hydrosilylation reaction to an organic compound that includes one carbon-carbon double bond per molecule and a cyclic polyorganosiloxane having at least three SiH groups per molecule.
For example, the following methods are generally known as methods for producing the cyclic polyorganosiloxane:
(a) carrying out hydrolysis and condensation to an organosilane having two hydrolysable groups that bind to silicon (e.g., Patent Literatures 1 and 2);
(b) heating a chain polyorganosiloxane in the presence of water and activated clay, to obtain a cyclic polyorganosiloxane (e.g., Patent Literature 3);
(c) heating a chain polyorganosiloxane and an acid catalyst, to obtain a cyclic polyorganosiloxane (e.g., Patent Literature 4); and
(d) heating a chain polyorganosiloxane and a neutral metal alkoxide catalyst, to obtain a cyclic polyorganosiloxane (e.g., Patent Literature 5).